The main application of scandium (Sc) metal is as a minor alloying additive in a number of aluminium (Al) alloys which are used in aerospace components and luxury and high performance sports equipment. Despite the high of cost of scandium, its field of application in Al alloys is gaining increasing interest due to the attractive characteristics it induces when added to these alloys. The use of Al—Sc alloys is of particular interest in the aerospace industry because the addition of Sc improves the weldability of the Al alloy. This may enable such alloys when used as skin components in aerospace applications to be joined by welding rather than the more expensive riveting techniques that are currently employed. However, expanding the current limited market of Al—Sc alloys depends on reducing the cost of scandium and establishing secure and reliable production and processing routes.
Al—Sc alloys have fine Al3Sc precipitates that are coherent with the Al matrix. The Al3Sc precipitates tend to affect several of the alloy's characteristics, including strength, weldability and recrystallization behaviour. There are three main effects that can be obtained by adding Sc to Al alloys: (i) grain refinement in heat affected areas during casting or welding, (ii) precipitation hardening from Al3Sc particles and (iii) grain structure control from Al3Sc dispersoids. Addition of Sc to Al improves its weldability by reducing recrystallization and limiting excessive grain-growth in heat affected weld zones. Also, the presence of fine dispersions of Al3Sc has been shown to increase both the strength and the creep resistance of coarse-grained binary Al alloys in addition to providing excellent fatigue properties with the resulting alloys amenable to be cold forged, hot forged or cast using vacuum die casting.
Addition of some other metals in combination with Sc can amplify the advantageous effects of Sc on Al alloys; for example, it is known that the use of zirconium-scandium additives is particularly effective, due to the shell structure of the Al3(Sc,Zr) dispersoids. Zirconium (Zr) is known to increase both the strength and the recrystallization resistance of Al—Sc alloys by substitution of Sc for Zr to form Al3(Sc(1-x)Zrx) precipitates with decreased coarsening kinetics in comparison to Al3Sc. Simultaneous addition of Sc and Zr has been shown to synergistically promote much higher strengths than either Sc or Zr additions produce alone. However, current techniques for the production of Al—Sc alloys have faced a number of difficulties. Alloying Sc metal directly with aluminium melt is slow and can require extended times to dissolve the bulk Sc metal or Sc master alloy lumps unless the melt is heated to more than 1150° C. Furthermore, this method requires the additional cost of producing high purity Sc metal. The most common scandium purification processes starts by producing scandium halides (e.g. scandium chlorides) which are then converted to oxides and then to the metal.
Another method of adding Sc to Al involves reducing high purity scandium oxide directly in the Al melt to produce an Al—Sc ingot.
It would be desirable to have a low cost process for production of Al—Sc based alloys and master alloys preferably in a powder form. Such a process would be particularly useful if it enables formation of compounds that cannot be obtained using current melt routes where the constituting elements are not chemically compatible.